For reasons of profitability, the cultivation of useful plants in agriculture or in horticulture aims at achieving high crop yields and at achieving this with as little input of material, such as fertilizers and pesticides, and as little expenditure of labor as possible. Where environmentally friendly cultural methods are employed in order to produce so-called bioproducts, an additional factor is that the use of chemical fertilizers or plant protectants has to be largely or entirely dispensed with.
An increase in yield may generally be achieved by increasing the volume of the harvest crop. This, in turn, can be achieved by promoting flower formation, by promoting fruit set and by increasing the size and weight of the individual pieces of fruit.
After flowering or after fruit setting, flower drop or premature fruit drop occur frequently due to atmospheric influence; this further reduces the number of fruits. Hence, an increase in crop yield can also be affected by inhibiting flower drop or fruit set.
The term “fruit set” is generally understood to mean the percentage of flowers which have begun to develop into a fruit. Generally, the fruit set is 10 to 30%, depending on the year, the species, the variety, the flower density and the weather conditions.
Particularly in fruit trees it is frequently observed that a very heavy flowering is followed by an insufficient fruit set. The physiological processes underlying this phenomenon are still largely unclear. Fruit set is determined on the one hand by the physiological condition of the plant or fruit free, on the other hand it is also determined by the weather conditions during flowering, which are of crucial importance for pollen transmission (pollination) and for the viability and further development of the seed buds.
Measures adopted to promote fruit set are chemical or mechanical thinning methods (cutting measures) to thin the flowers, or dressing of leaves with nitrogen-containing fertilizers.
The above-mentioned thinning methods are disadvantageous as they cause additional expenditure of labor and additional costs; in addition, although the thinning promotes the fruit set in the remaining flowers, this is at the cost of the flowers removed by the thinning. The above-mentioned foliage dressing likewise involves additional expenditure of labor and additional costs; in addition, this measure cannot be used in ecological methods of cultivation.
A further problem in the cultivation of useful plants for producing fruits, particularly in orcharding, consists in that the plants exhibit increased vegetative growth, that is, an increased shoot growth with increased growth of herbage. In fruit trees, this growth usually starts with flowering and lasts up to approximately the middle of July; it ends with the closing of the buds.
This increased vegetative growth can lead to a reduction of the crop yield, as well as—owing to the formation of excessive shade—to detrimental effects on the fruit quality. Excessive shoot growth or longitudinal growth of the plants, especially of fruit trees, is also undesirable because it renders harvesting more difficult. Furthermore, excessive longitudinal growth can lead to the plants being damaged by wind, rain or hail since they are not able to withstand the mechanical stress. This particularly applies to rape and grain.
For this reason, repeated cutting measures or treatments with synthetic growth inhibitors are required in order to counteract this excessive growth. These cutting measures are very labor- and cost-intensive measures.
By using synthetic growth inhibitors, the extent of the cutting measures can be reduced. Several growth inhibitors suitable for this purpose are known, e.g. chlormequat, mepiquat chloride, triazole compounds, such as paclobutrazol, or acylcyclohexane diones, such as trinexapacethyl. It is disadvantageous, however, that these growth inhibitors have to be applied in high dosages per hectare and that they present a potential risk to the environment. For ecological or near-natural production of fruits, the use of such growth inhibitors is completely out of the question. This is also true of more recent types of growth inhibitors, which are active at lower dosages. Abscisic acid (ABA) is a phytohormone which in perennial ligneous plants is responsible for the initiation and maintenance of bud dormancy during the dormant period in winter. ABA is furthermore said to have an influence on the water balance of the plant, and it can inhibit seed germination. ABA is generally regarded as the stress hormone of plants, that is, ABA is produced above all under stress conditions (e.g. drought, or temperature-induced stress). As early as 1963 it was reported that endogenously produced ABA is responsible for leaf abscission in cotton. Under conditions of drought, ABA affects the closure of the stomata, which prevents further loss of water and thereby prevents that the plant dries up.
International Publication No. WO 03 096806 A2 mentions that ABA can cause defoliation (leaf abscission), fruit drop and dormancy, and that, in fruit trees, it is said to prevent resprouting caused by certain weather conditions. There are no details given as to the mode of application; in particular, the time of application and the dosage are not indicated.
International Publication No. WO 03 096806 A2 furthermore describes the use of ABA to prevent the cracking of cherries, to increase frost hardiness and to increase the sugar content in fruit.
U.S. Pat. No. 5,173,106 describes the use of ABA to inhibit flowering and to extend the duration of flowering in flowering plants.